Updating start date and end date at header level for a contract

As per the requirement I have to create a contract and assign the start date and end date to the contract.

I am using BAPI_CONTRACT_CREATEFROMDATA for creating a contract. I am assigning value for start date and end date for header data of contract in fields

-CT_VALID_F and CT_VALID_T of CONTRACT_HEADER_IN import parameter and also setting the corresponding indicator in CONTRACT_HEADER_INX parameter.

The BAPI executes successfully and does not return any error but when I check the start date and end date for contract in va43 the values are not assigned.

I also want to update the start date and end date for contract in other case for which I am using BAPI_CUSTOMERCONTRACT_CHANGE and passing date values to the same fields as those create but it give me an error while executing. The error is as follows:

Field 'GUEEN' cannot be changed, VBAKKOM 2000000162 ready for input.

Can anyone suggest what can be done for update the date values?

Thanks in advance.

Komal